1. Field of the Invention
The present invention relates to a solid polymer electrolyte, particularly three-dimensionally isotropic solid polymer electrolyte of high ionic conductivity.
2. Description of the Related Art
The advantages of a solid electrolyte have long been recognized for rechargeable energy storage, i.e., battery and super-capacitor. Solid electrolytes are deemed to have superior energy density, cycle lifetime as well as volumetric, electrochemical and environmental stabilities unachievable by liquid or gel-type electrolytes. In addition, solid electrolyte permits significant improvements by allowing separator-free, unrestricted orientation and large surface-to-thickness device structure. Solid polymer electrolytes are conceived to have unique benefits in chemical variety, energy density, thermal stability, lightweight, flexible film, simple processing, low memory effect and long shelf life. Currently, solid polymer electrolytes for battery applications are limited by their low electrical conductivity. The room-temperature conductivity for the state-of-the art solid polyelectrolytes, such as LiSO3CF3 doped poly(ethylene oxide), is about 10−8 to 10−6 S/cm (V. Chandrasekhar, Adv. Polym. Sci. 1998, 135, 139) which is at least three orders of magnitude smaller than practical battery requirements.
A recently developed rigid-rod molecule of poly-p-phenylenebenzobisimidazole was a sulfonated heterocyclic aromatic polymer of poly[1,7-dihydrobenzo[1,2-d:4,5-d′]diimidazo-2,6-diyl-(2-sulfo)-p-phenylene]], abbreviated as sPBI (J. F. Wolf and F. E. Arnold, Marcromolecules 1981, 14, 909; J. F. Wolf, B. H. Loo and F. E. Arnold, Macromolecules 1981, 14, 915). The chemical structure of sPBI is illustrated in the following formula IIa, 
and y represents an integer larger than 10.
Because of its large molecular aspect ratio (length/width) from para-catenated backbone, this polymer assumes a nematic liquid-crystalline state having a fully conjugated backbone with configurations depending only on the rotation of bonds between alternating phenylene and heterocyclic groups (W. J. Welsh, D. Bhaumik and J. E. Mark, Macromolecules 1981, 14, 947). Consequently, this rod-like molecule displays superior mechanical tenacity, thermo-oxidative stability, and solvent resistance preventing thermal molding or extrusion to be applied to process sPBI. This intractable polymer is generally fabricated using an acidic solvent, most commonly methanesulfonic acid (MSA) or Lewis acids.
Chemical derivatization of sPBI with pendants of propane-sulfonate ionomers for a sPBI-X+ polyelectrolyte has been demonstrated successfully to promote solubility of sPBI (J. J. Fitzgerald and R. A. Weiss, J. Macromol. Sci.-Rev. Marcomol. Chem. Phys. 1988, C28, 99; M. Aldissi, U.S. Pat. No. 155,450, 1989; T. D. Dang and F. E. Arnold, Mater. Res. Soc. Sym. Proc. 1993, 305, 49). As reported (T. D. Dang, S. J. Bai, D. P. Heberer, F. E. Arnold and R. J. Spry, J. Polym. Sci., Part B Polym. Phys. 1993, 31, 1941), sPBI-X+ rigid-rod polyelectrolyte did show drastically enhanced water solubility and electrical conductivity. The conductivity was further identified to be ionic and not electronic, a critical factor for solid polyelectrolyte applications. However, it was also reported that cast films of sPBI-X+ from a solution had structural anisotropy leading to anisotropic electric conductivity. The room-temperature direct-current (DC) conductivity parallel (σ∥) and perpendicular (σ⊥) to the film surface could be 8.3×10−3 S/cm (T. D. Dang, S. J. Bai, D. P. Heberer, F. E. Arnold and R. J. Spry, J. Polym. Sci., Part B Polym. Phys. 1993, 31, 1941) and about 10−6 S/cm (R. J. Spry, M. D. Alexander, S. J. Bai, T. D. Dang, G. E. Price, D. R. Dean, B. Kumar, J. S.  Solomon, and F. E. Arnold, J. Polym. Sci., Part B: Polym. Phys. 1997, 35, 2925), respectively. This anisotropic conductivity was attributed to the entropy effect in casting films of nematic liquid-crystalline rigid-rod polymers of sPBI and sPBI-X+ (P. J. Flory, Proc. R. Soc. London Ser. A 1956, A234, 73). On the other hand, the room-temperature conductivity of battery, fuel cell or super-capacitor should be up to 10−4 S/cm. As a result, the room-temperature direct-current (DC) conductivity perpendicular (σ⊥) to the film surface fails to meet the required conductivity for battery, fuel cell or super-capacitor.